Part 10.8 AROMATIC HYDROCARBONS (ARENES) -
introduction to arene electrophilic substitutions. Chlorination/bromination to aromatics like
chlorobenzene, bromobenzene etc. The orientation of products in aromatic substitution
(1,2-; 1,3-; and 1,4-
positions for two substituents in the benzene ring, old names -
ortho/meta/para substitution products). The revision
notes
include full diagrams and explanation of the mechanisms and the 'molecular' equation and reaction conditions
and other con-current reaction pathways and products are also explained
for the reaction mechanisms of aromatic hydrocarbons like benzene and
methylbenzene.

The five
reactions described are electrophilic substitution reactions involving the
generation of a powerful electrophile (electron pair acceptor) which
subsequently attacks the electron rich Π (pi) electron system of the double bond. Arenes tend
to undergo substitution, rather than addition, because substitutions allows
the very stable benzene ring to remain intact.

Chlorine is bubbled
into a mixture of the arene and anhydrous aluminium chloride catalyst.
Other catalysts like anhydrous iron(III) chloride can be used, and they
are collectively known as halogen carriers.

Step
(1) The non-polar and uncharged
chlorine molecule is not a strong enough an electrophile to disrupt
the
pi electron system
of the benzene ring. The aluminium chloride reacts with a chlorine
molecule to form a positive chlorine ion Cl+
which is a much stronger electron pair accepting
electrophile and a tetrachloroaluminate(III) ion (either this or an
AlCl3-Cl2 complex - details not needed for A
level).

Step
(2)An electron pair from the
delocalised
pi electrons of the
benzene ring forms a C-Cl bond with the electron pair accepting
positive chlorine ion forming a highly unstable carbocation. It is
very unstable because the stable electron arrangement of the benzene
ring is partially broken to give a 'saturated' C (top right of
ring).

when R = CH3,
methylbenzene forms a mixture of chloro-2/3/4-methylbenzene.

chloro-3-methylbenzene is the minority product and the mechanism above
would show the formation of chloro-2-methylbenzene.

FURTHER COMMENTS

The overall
halogenation
reaction is the substitution of -H by -Cl

Bromination
can be carried in the same way by mixing bromine, the aromatic
hydrocarbon (arene) with a halogen carrier catalyst such as
anhydrous AlBr3
or FeBr3.

AND you can write out the
mechanism in exactly the same way, but putting in Br instead of Cl.

Why do aromatic
compounds tend to react by electrophilic substitution BUT
alkenes tend to react by electrophilic addition?

They both
interact with electrophiles because they both have 'electron
rich' electron pair donating bonding systems i.e. the >C=C<
double bond in alkenes and the delocalised ∏electrons of
the benzene ring, but the benzene ring has a particularly high
stability which is preserved on substitution. For the same
reason alkenes are generally more reactive than arenes.

If methyl benzene
is reacted with chlorine in the presence of uv light, substitution
takes place in the alkyl side chain. In other words it behaves like
an alkane and undergoes a free radical substitution reaction.
The initial product is chloromethylbenzene, C6H5CH2Cl,
and further substitution products can be formed C6H5CHCl2
and C6H5CCl3. This illustrates the
significance of changing reaction conditions which function via a
different mechanism to give a different product.

10.8.7 The
orientation of products in aromatic
electrophilic substitution reactions

Certain
groups, already present, can increase the electron density of
the benzene ring and make the aromatic compound more reactive
towards electrophiles such as those described above. However
the effect seems to enhance the reactivity at the 2 and 4
substitution positions more than the 3 substitution position.

They all,
by some means, have a small, but significant, electron
donating (+I inductive effect) on the ring of
pi
electrons.

For
example, methyl benzene is significantly more reactive
than benzene and when nitrated, over 90% of the products
are either methyl-2-nitrobenzene or methyl-4-nitrobenzene.

Certain
groups, already present, can decrease the electron density of
the benzene ring and make the aromatic compound less reactive
towards electrophiles such as described above.
However the effect seems to decrease the reactivity at the 2 and
4 substitution positions more than the 3 substitution position.

Groups
that decrease reactivity, by some means, are e.g.
-NO2, COOH, -CHO, -SO2OH, and favour
substitution at the 3 position (typically 70-90%) and their
effect does fit in with them all being strongly
electronegative groupings giving a
-I inductive effect.

For
example, nitrobenzene is much less reactive than benzene
and on nitration, 93% of the product is 1,3-dinitrobenzene.